Conventional CCD image sensors are illustrated in FIGS. 8 to 11.
FIG. 8 is a plan view showing the general configuration of a conventional two-dimensional CCD image sensor. The sensor comprises photoelectric converters, in the form of photo diodes 101, vertical registers 102, read-out gates 103, a horizontal register 104, and an output 105.
Signal charges converted by a photoelectric converter are stored for a charge storage period (for example, 1/60 seconds). The read-out gate being driven opens to read out the stored signal charges to the adjacent vertical register (see arrows A). During a horizontal blanking period, the signal charges are transferred toward the horizontal register (see arrows B). Subsequently, the horizontal register transfers the signal charges toward the output at a high speed (see arrow C) during a horizontal scanning period. The period between the first read-out of charges to the next is the charge storage period.
FIG. 9 is a plan view of a portion of conventional vertical CCD registers, which is referred to in explaining configuration of vertical CCD registers 102. Toshio NOBUSADA et al. reported on a frame interline transfer CCD image sensor that uses a poly-Si/Al double-layer transfer gate and P+floating island isolation "Frame Interline Transfer CCD sensor for HDTV Cameras" Digest of Technical Papers, pp. 88-89, IEEE International Solid-State Circuit Conference Feb. 15, 1989. The configuration illustrated is FIG. 9 is analogous to the poly-Si/Al double layer transfer gate illustrated in pp. 89 of the Digest of Technical Papers. In FIG. 9, transfer electrodes 106 are successively arranged in each vertical CCD channel (not illustrated) a short distance apart from each other to transfer signal charges. A metal wiring layer 107 is connected to the transfer gates 106 to apply drive pulses. The metal wiring layer 107 is connected to the transfer electrodes 106 via contact apertures 108. The transfer electrodes 106 of one vertical register and those of the other vertical registers are interconnected via wiring and they are dividable into a number of groups each for one line of photodiodes 101 such that each group of transfer electrodes is adapted to be subject to a train of pulses in common.
In the example shown in FIG. 9, the wiring layer 107 includes metal wires 107a, 107b, 107c and 107d, and each metal wire has one contact window or aperture 108 every four successively arranged transfer electrodes 106 belonging to one vertical register. The adjacent four metal wires 107a, 107b, 107c and 107g have contact apertures 108 opening to four different transfer electrodes 106, respectively, which are in successively arranged four lines and belong to successively arranged four vertical registers, respectively. Thus, each metal wire 107a, 107b, 107c and 107d is connected to one transfer electrode every four successively arranged transfer gates 106 of one vertical register. Four different in phase trains of pulses are applied to the metal wires 107a, 107b, 107c and 107d, respectively.
This connection between the transfer electrodes 106 and the metal wiring 107 is required to accomplish transfer of signal charges through the CCD channels at high frequency. In the example of Figure, the transfer electrodes 106 are formed of poly-Si, which has high interlayer resistance in the order of several tens Ohms. This high resistance brings about signal distortion of drive pulses if driven at a frequency exceeding 20 MHz, thus making it difficult to transfer charges. The provision of the contact apertures 108 to establish points at which the transfer electrodes 106 are connected to the metal wiring 107 is found to be effective in suppressing the signal distortion of drive pulses at transfer electrodes 106, thereby to allow pulses to be driven at a high frequency. Let us consider one group of transfer electrodes for one line of photodiodes, which are interconnected by the metal wiring. These transfer electrodes of one group are connected to a portion of the metal wiring supplied with the same pulse voltage every four CCD channels. They are subject to the same pulse voltage at intervals of several tens microns, making it possible to drive pulses at a sufficiently high frequency. In order to reduce photo-generated noise charges due to incident light to the CCG channels, a photo-shield layer is used as the metal wiring 107.
FIG. 10 is a cross section taken though the configuration of FIG. 9 along a direction of transfer of signal charges across a CCD channel through one CCD register. In FIG. 10, deposited on N-type substrate 110 is P.sup.- -type well 115. Deposited on the P.sup.- -type well 115 is N.sup.- -type impurity layer 116 that severs as CCD channel. A gate insulating layer 121 is deposited on the N.sup.- -type impurity layer 116. Poly-Si transfer electrodes, generally designated at 106, 106a, 106b, 106c and 106d are deposited on the insulating layer 121. Al wiring 107 is deposited on the insulating layer 121 over the transfer electrodes 106a, 106b, 106c and 106d and it has contact apertures 108. The contact apertures 108 are located above gate regions of transfer electrodes 106d. In FIG. 10, one in every four transfer electrodes is connected to the same Al wiring in common. That is, the transfer electrodes 106d arranged across three transfer electrodes 106a, 106b and 106c are connected to the same Al wiring 107 in common.
FIG. 11 is a plan view illustrating one unit pixel of the configuration of FIG. 9. The reference numeral 101 designates a photodiode. The reference numeral 205 designates a vertical CCD channel. The reference numeral 103 designates a read-out gate. The reference numeral 118 designates a diffusion region for separating the photodiode 111 from the CCD channel 103. The reference numerals 210 and 220 designates a first poly-Si electrode layer used as a transfer electrode and a second poly-Si electrode layer used as a transfer electrode, respectively. The reference numeral 108 designates a contact aperture used to connect the transfer electrode 220 to the Al wiring 107, not shown in FIG. 11. The CCD channel 205 separated by the diffusion region 118 allows transfer of signal charges. The contact aperture 108 is located above the gate region of the transfer electrode and on the centerline of the CCD channel 205. Kazuya YONOMOTO et al. reported on a CCD image sensor with contact apertures on the centerline of each CCD channel "A 2 Million Pixel FIT-CCD Image Sensor for HDTV Camera System" Digest of Technical Papers PP. 214-215, IEEE International Solid States Circuits Conference, Feb. 16, 1990.
The electrode configuration shown in FIG. 9 exhibits a drop in transfer efficiency down from a satisfactorily high level. This drop is derived from the provision of contact apertures because, without the provision of contact apertures, this phenomenon was not found. Specifically, it is derived from contacts between the poly-Si transfer electrodes 106 and the Al wiring 107.
Experiment was conducted with one case where contact apertures were located within active region of poly-Si gate electrode of MOS type transistor and other case where no contact apertures were located within active region of poly-Si gate electrode of MOS type transistor to compare threshold voltages with respect to the two cases. FIG. 12 shows the results of this experiment. Tungsten was used as the metal wiring. As is appreciated from FIG. 12, a drop in threshold voltage by about 1 (one) volt was observed if the contact apertures were provided. This is because potential under the transfer electrode dips at an area where the contact aperture is located, thereby to create a dipping in the potential well.
FIG. 13 illustrates distribution of potential along a cross section taken through the line X--X in FIG. 11, illustrating a dipping in potential below an area where the contact aperture is located. The dipping in the potential well will trap a portion of charges, thereby to cause a drop in transfer efficiency. It is not yet clarified what brings about such a dipping in potential at an area where a contact aperture is located. The depth of such dipping is dependent on the material used as the metal wiring. Therefore, the direct contact between the poly-Si electrode and the metal wiring is considered to play some important role in creating a dipping in potential.
Judging from this point of view, it is preferred to use a CCD register configuration as shown in FIG. 14 where no direct contact between the transfer electrode and the metal wiring is used. In FIG. 14, the same reference numerals as used in FIG. 10 are used to denote like or similar parts or portions to those used in FIG. 10. In FIG. 14, the reference numeral 211 designates a poly-Si film used as a wiring electrode. The reference numeral 123 designates an interlayer insulating film. The reference numeral 201 designates a first contact aperture. The first contact aperture 201 interconnects a transfer electrode assembly 106 (106a, 106b, 106c, 106d) and the poly-Si wiring electrode 211. The reference numeral 202 designates a second contact aperture. The second contact aperture 202 interconnects the poly-Si wiring electrode 211 and the metal wiring 107. It may be noted that the first and second contact apertures 201 and 202 are located at different areas, respectively.
According to this configuration shown in FIG. 14, the metal wiring 107 is not in direct contact with the transfer electrode 106 because the poly-Si transfer electrode assembly 106 is connected to the poly-Si wiring electrode 211 via the first contact aperture 201 and the poly-Si wiring electrode 211 is connected to the metal wiring 107 via the second contact aperture 202. The first and second contact apertures 201 and 202 are located at different areas spaced from each other in the horizontal plane. This configuration has proven to be effective in preventing occurrence of any dipping in potential.
However, the addition of poly-Si wiring electrode 211 and second contact aperture 202 causes increased complexity in wiring. The complicated wiring makes it very difficult to use the CCD register in constructing a CCD image sensor with highly condensed pixels. Increased surface irregularities under the metal wiring may damage the wiring and a covering layer. Any damage on the covering layer causes smearing to occur.
An object of the present invention is to provide a CCD image sensor, which has eliminated the above-mentioned drop in signal charge transfer without any increase in complexity of electrode configuration and without any increase in smear level.